Looking through my development folders, I found something odd - an old model for "tartane50_mast4.gm". As far as I can see, the only difference between that version and the current one is that the old one has a geometry locator for "basket", and indeed there is still "tartane50_Basket.gm" included in the folder "RESOURCE\MODELS\Ships\Tartane50".
Watch the sequence of Captain Jack Sparrow entering Port Royale, either by re-watching the film, searching YouTube, or just watching the game video "RESOURCE\VIDEOS\LegendJackSparrow\jack_entrance.wmv". Or just look at this:
The "real" Jolly Mon does have a basket on the mast. It's a hollow frame, whereas "tartane50_Basket.gm" is solid and has a lid, and appears to be a duplicate of the one which may appear among the flotsam of a sunken ship. So the model is not accurate but it's better than nothing, which is what "Tartane50" has at the moment.
Then there's the flag. It's not an ensign, it's a pennant - it may not be as long and streamy as the pennants seen on some models of larger ships, but it's longer, streamier and forkier than any ensign should be, and more so than some of the pennants seen on older models. Indeed, the standard pennant design does not cut deeply enough between the tails to depict this one, but that's easily handled by playing with the alpha channel. So then the question becomes, where to put the texture? Jack Sparrow already has two personal flags and as we're running short of personal flag slots, he's not getting another. So this one can replace the pennant for either his British flag (St. George's cross plus red bird) or non-British flag (blue flag with yellow bird).
As an experiment, I replaced the non-British pennant, put the original mast model back into place, edited "Ships_init.c" to make "Tartane50" use a pennant instead of an ensign, and this is the result:

Watch the sequence of Captain Jack Sparrow entering Port Royale, either by re-watching the film, searching YouTube, or just watching the game video "RESOURCE\VIDEOS\LegendJackSparrow\jack_entrance.wmv". Or just look at this:

The "real" Jolly Mon does have a basket on the mast. It's a hollow frame, whereas "tartane50_Basket.gm" is solid and has a lid, and appears to be a duplicate of the one which may appear among the flotsam of a sunken ship. So the model is not accurate but it's better than nothing, which is what "Tartane50" has at the moment.
Then there's the flag. It's not an ensign, it's a pennant - it may not be as long and streamy as the pennants seen on some models of larger ships, but it's longer, streamier and forkier than any ensign should be, and more so than some of the pennants seen on older models. Indeed, the standard pennant design does not cut deeply enough between the tails to depict this one, but that's easily handled by playing with the alpha channel. So then the question becomes, where to put the texture? Jack Sparrow already has two personal flags and as we're running short of personal flag slots, he's not getting another. So this one can replace the pennant for either his British flag (St. George's cross plus red bird) or non-British flag (blue flag with yellow bird).
As an experiment, I replaced the non-British pennant, put the original mast model back into place, edited "Ships_init.c" to make "Tartane50" use a pennant instead of an ensign, and this is the result:
